Position detector

ABSTRACT

A detector for checking the symmetry of a helicopter rotor while it is rotating has a scanner (11) which scans vertically a field of view (15) and the view is interrupted by a vertical strip on the blade tip (16). A slot (18) in the strip gives a characteristic signal as at (L 2 ). The position of the signal (L 2 ) in a scanning cycle gives an indication of the height of the blade tip and also the circumferential spacing between blades.

BACKGROUND OF THE INVENTION

This invention relates to a position detector, for example for detectingthe position of a movable object but one example is to detect thevertical position of the tips of rotor blades of a helicopter rotor asthey rotate so that any lack of uniformity in the deflection of all theblades in the rotor can be detected and corrected.

Although the invention will be particularly described as applied to thatuse, it will be appreciated that it can be used for detecting theposition of other articles, for example the vertical deflection of theend of a stationary cantilever beam or the deflection of rotor bladetips in a turbine, or components of other rotating machinery.

SUMMARY OF THE INVENTION

The invention has various aspects, but from one point of view can beconsidered to embody a position detector comprising means defining anelongate field of view and a target which may carry a reference mark andis capable of being in any of a range of positions along the field ofview, and a sensor arranged to scan along the field of view.

The target may be fixed to an object for example a rotor blade whoseposition is to be detected, and the target itself may be elongateparallel with the field of view, which in turn will extend in thegeneral direction of expected deflection. The reference mark can be aline extending transversely to the elongate target and could be a slotin an opaque target so that light can be seen through the slot by thesensor. Again the reference mark could be a highly reflective strip on anon-reflective target surface, but that application would require asource of light or other radiation to which the sensor is sensitive forilluminating the reference mark when the target was in the field ofview.

The advantage of having a reference mark on the target is that the widthof the reference mark and its position along the target can be preciselymeasured, and that can provide reference information from which theactual deflection of the article carrying the target can be determined.Alternatively the blade itself could act as the target, preferably thetrailing edge of the blade, as this would be lower than the leading edgewhen the pitch of the blade is adjusted for flight conditions.

The means defining the elongate field of view could comprise an elongatelens.

The sensor could consist of a multi-element linear sensor withelectronic means for scanning the elements in a scanning cycle.

In the application to a helicopter rotor, a target could be positionedon the tip of each rotor blade, extending in the vertical direction withan elongate slot constituting the reference mark of a known width at aknown distance from the end of the target strip, and with the field ofview and the sensor extending parallel to the length of the target.

The invention is quite simple because for daylight use it is notnecessary to irradiate the target, and the sensor can be coupled tosolid state logic circuits capable of giving a direct indication of theamount of deflection of a helicopter rotor blade or other article whichinformation can be updated as it changes. Of course other types of logiccircuit could be used if desired.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF DRAWINGS

The invention may be carried into practice in various ways, and oneembodiment will now be described by way of example, together with avariation, with reference to the accompanying drawings, in which

FIG. 1 is a diagrammatic front elevation of a helicopter using a sensorin accordance with the invention;

FIG. 2 is a detail to an increased scale of the tip of a helicopterblade as seen in the direction of the arrow II in FIG. 1;

FIG. 3 is a block schematic diagram of the circuit using the sensor ofFIG. 1; and

FIGS. 4, and 5 are characteristics which arise during use of the sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The sensor is for providing an indication of the deflection of theindividual blades of a rotating helicopter rotor, and also for providingan indication of any circumferential lag or lead of an individual blade.

The sensor embodies a charge coupled device (CCD) 11 which comprises anormally vertical line of sensor elements (256 in the example beingdescribed) including an integrated circuit embodying a shift register,so that the electrical charge on each element can be transmitted by wayof a control 12 (FIG. 3) to a microprocessor 13, as the elements arescanned in a scanning cycle controlled by the shift register. In frontof the CCD is a lens 14, which is arranged to focus onto the verticalline of sensor elements, light from a narrow vertical strip field ofview indicated generally at 15 in FIGS. 1 and 2. The strip might beabout 5 centimeters high at the tip 16 of the helicopter blades, one ofwhich is shown in FIGS. 1 and 2.

The lens inverts the image of the field of view 15 and the elements inthe CCD are scanned vertically downwards by the shift register so thatthe image received is that of a thin vertical slice of background, whichwill be interrupted regularly by a rotating rotor blade.

For calibration purposes, the tip of each blade carries a dependingtarget strip 17 perhaps 10 centimeters high and 3 centimeters widehaving at a known distance `L` (perhaps equal to one centimeter) up fromthe lower edge, a horizontal rectangular hole `L` high and nearly aswide as the target strip 17.

The effect will be that during a scan of the CCD while the field of view15 is interrupted by a blade and its target 17, the signal received bythe microprocessor will be as shown in FIG. 4 consisting of a highsignal representing peak white for a time `t` corresponding to viewingof the clear background below the target, and then a period `L`, of lowsignal represented by the obscuring of light by the bottom of thetarget, then an equal length period `L₂ ` of peak white signal as lightis received through the hole 18, and finally a low signal black perioduntil the end of the time `T` of the scan because of the obscuring oflight first by the top part of the target 17 and then by the body of theblade.

It will be appreciated that the time `t` before the black signal shownin FIG. 4 will be a measure of the absolute vertical displacement of thelower edge of the target, and hence of the blade in relation to somereference height. The scanning time for the CCD 11 will be shorter thanthe time it takes for the blade and target to cross the field of view15, so that several scans are made during the passage of each blade, thelowest reading being taken as the true blade position. The signals fromthe various blades can be compared so that any variation in the heightsof the individual blade tips can be indicated.

The circuit of FIG. 3 includes a number of timing counters 19 used forgiving signals representing the quantities `t`, `L₁ `, and `L₂ `, asshown in FIG. 4. L₁ and L₂ correspond to the known equal heights of thelowest opaque portion of the target and the hole 18 respectively, and byaveraging those values, compensation can be obtained for any errorsintroduced by any change in the sensing threshold of the logic caused bydifferent levels of lumination. The height corresponding to `L` isknown, so that `t` can be accurately determined in terms of that knownheight `L`. During initial calibration, the blades need not be rotating.

A reference pulse is fed into the circuit of FIG. 3 once per revolutionof the rotor, and by relating the signals such as shown in FIG. 4 foreach of the rotor blades to that reference pulse, any variation of thecircumferential position of any blade from the desired equalcircumferential spacing can be indicated even if a variation occurs fromrevolution to revolution.

A control circuit 22 monitors the peak video output from the CCD sensor,and one output from this is used to control an iris 21 which is part oflens 14. Thus, the peak output from the CCD sensor is maintained forvarying light levels.

A second output from the control circuit 22 informs the microprocessor13 if the light level is too low for satisfactory operation. Themicroprocessor can then increase the integration time and/or inform theoperator that the light level is too low.

An output from the microprocessor representing `t/T` is converted intoan analogue signal in a convertor 24 to drive a meter 25 which isconveniently set so that when the meter indicator is at half full scaledeflection the image of the hole 18 is in the centre of the field ofview 15.

Information from the microprocessor 13 is also sent by way of a serialdata interface 26 to a computer which can display the detectedinformation visually.

For use of the system at night, when there is insufficient backgroundillumination, the target 17 can be replaced by one having instead of thehole 18 a highly reflective strip of the same shape facing the camera,and arranged to reflect illumination from a lamp 27 which can in fact becontrolled from the microprocessor 13 through a driver 28 to beilluminated only when a blade is due to enter the field of view 15.

The signal received is inverted and so will appear as shown in FIG. 5being "black" for most of the cyclic period `T` but being "peak white"for an interval `L` corresponding to the reflective strip at the time`t` from the start of the scan.

I claim: .[.1. A position detector comprising means defining an elongatefield of view, for viewing a target which is arranged to cross the fieldof view in any of a range of positions along the length of the field ofview and having a reference part, a sensor in the form of a line ofsensitive elements extending parallel with the field of view andpositioned to receive an image of the field of view, and electronicmeans for repeatedly scanning the elements in scanning cycles and fordetecting the particular sensitive element which receives the image ofthe reference part of the target as the target crosses the field ofview..]. .[.2. A detector as claimed in claim 1, in which the target isfixed to a rotor blade, whose position is to be detected..]. .[.3. Adetector as claimed in claim 1, in which the target comprises a blade ofa helicopter rotor and the reference part is the edge of the blade..].4. .[.A detector as claimed in claim 1, in which the.]. .Iadd.A positiondetector comprising means defining an elongate field of view, forviewing a target which is arranged to cross the field of view in any ofa range of positions along the length of the field of view and which hasa reference part, which said .Iaddend.reference part is a slot extendingperpendicularly to the length of elongation of the field of view.Iadd.,a sensor in the form of a line of sensitive elements extending parallelwith the field of view and positioned to receive an image of the fieldof view, and electronic means for repeatedly scanning the elements inscanning cycles and for detecting the particular sensitive element whichreceives the image of the reference part of the target as the targetcrosses the field of view.Iaddend..
 5. .[.A detector as claimed in claim4, in which.]. .Iadd.A position detector comprising means defining anelongate field of view, for viewing a target which is arranged to crossthe field of view in any of a range of positions along the length of thefield of view and which has a reference part, which .Iaddend.saidreference part is a reflecting line and further including a source ofradiation to which the sensor is sensitive for illuminating thereflecting line when the target is in the field of view.Iadd., a sensorin the form of a line of sensitive elements extending parallel with thefield of view and positioned to receive an image of the field of view,and electronic means for repeatedly scanning the elements in scanningcycles and for detecting the particular sensitive element which receivesthe image of the reference part of the target as the target crosses thefield of view.Iaddend..
 6. .[.A detector as claimed in claim 1, in whichthe.]. .Iadd.A position detector comprising means defining an elongatefield of view, for viewing a target which is arranged to cross the fieldof view in any of a range of positions along the length of the field ofview and having a reference part, in which said .Iaddend.means definingthe elongate field of view comprises an elongate lens.Iadd., and saidreference part is a slot extending perpendicularly to the length ofelongation of the field of view, and further comprising a sensor in theform of a line of sensitive elements extending parallel with the fieldof view and positioned to receive an image of the field of view, andelectronic means for repeatedly scanning the elements in scanning cyclesand for detecting the particular sensitive element which receives theimage of the reference part of the target as the target crosses thefield of view.Iaddend.. .[.7. A detector as claimed in claim 1, furtherincluding computer means connected to receive the output from the sensorand including means for computing the position of the reference part inthe field of view..].
 8. .[.A detector as claimed in claim 7 incombination with.]. .Iadd.A position detector comprising means definingan elongate field of view, for viewing a target which is arranged tocross the field of view in any of a range of positions along the lengthof the field of view and which has a reference part, .Iaddend.ahelicopter rotor .Iadd.having blades constituting the target,.Iaddend..[.and having.]. .Iadd.a sensor in the form of a line ofsensitive elements extending parallel with the field of view andpositioned to receive an image of the field of view, and electronicmeans for repeatedly scanning the elements in scanning cycles and fordetecting the particular sensitive element which receives the image ofthe reference part of the target as the target crosses the field ofview, computer means connected to receive the output from the sensor andincluding means for computing the position of the reference part in thefield of view, and .Iaddend.means for generating a reference signal onceper rotor revolution, and in which the computer means includes means forcomputing the circumferential position of each rotor blade in relationto the reference signal.
 9. A method of testing a helicopter rotorhaving blades in which a thin, generally vertical elongate field of viewis defined, the rotor is rotated so that the blades in turn cross thevertical field of view, a multi-element sensor extending parallel withthe field of view is repeatedly scanned in scanning cycles, and theposition of the various blades in the field of view are computed fromsignals derived during the scanning cycles.
 10. A method as claimed inclaim 9, in which the scanning cycle is fast in relation to the timetaken for a blade to cross the field of view. .Iadd.11. The method ofclaim 10 wherein said elements are scanned several times each time thehelicopter blade crosses said field of view. .Iaddend. .Iadd.12. Themethod of claim 11 wherein the computation involves measuring the ratioof the time between the beginning of the scanning of said elements andthe instant when the first of the sensor elements upon which the imageof a rotor blade is projected is scanned and the time it takes to scanall of the sensor elements involved. .Iaddend. .Iadd.13. The method ofclaim 9 wherein said ratio is indicated by the position of a pointermoving over a scale. .Iaddend. .Iadd.14. The position detector of claim8 wherein said computer means includes means for computing the ratio ofthe time between the beginning of the scanning of said elements and theinstant when the first of the sensor elements upon which the image of arotor blade is projected is scanned and the time it takes to scan all ofthe sensor elements involved. .Iaddend. .Iadd.15. The position detectorof claim 14 combined with means for displaying said computed ratio, inwhich the reference part is a slot in an opaque target secured to arotor blade between the opposite ends of the target so that theprojection of said field of view on said sensor elements when the targetcomes into the said field of view includes a light area outside saidtarget, followed by the opaque area of said target in advance of saidslot, the slot area, and an opaque area of said target beyond the slot,each area being projected onto different elements in the line ofelements. .Iaddend. .Iadd.16. The position detector of claim 15 whereinsaid last mentioned means is a meter movement where said ratio isdisplayed by the position of a movable pointer on a scale. .Iaddend..Iadd.17. The position detector of claim 16 wherein there is providedmeans responsive to the peak output of the sensing elementscorresponding to the peak value of a number of measured values of thesaid ratio for varying the amount of light projected by said lightsource upon said field of view to keep the said peak output constant..Iaddend. .Iadd.18. The method of claim 9 or 12 wherein said signalsderived from successive scannings of said elements are averaged..Iaddend.